The invention relates to leak detectors and, in particular, to liquid or fluid-based leak detectors for testing for leaks in hydraulic or liquid-carrying valves, and the like.
Leak detection equipment is used in a variety of industries to determine whether products are properly manufactured and assembled. Leak detection equipment is used to test individual products for presence of leaks which would degrade the performance of the product during its useful life. Not all leaks are, however, fatal to the performance of the product and a maximum acceptable leak rate is often established.
The object of leak testing is to measure the rate of leakage and to determine whether the measured rate is less than the maximum acceptable leakage rate. Any product leaking at a rate less than the maximum acceptable leakage rate meets the performance specifications relating to the product.
A leak is defined as the escape or entry of a gas or liquid into sealed enclosure. Leaks may result from material defects due to holes or porosity or process deficiencies such as sealing and joining problems. The majority of leaks are not simple circular holes in thin walls exhibiting predictable performance but more often are comprised of multiple variable leak paths that tend to be unique to a particular test part subjected to a specific set of conditions. Leaks of these types often have low leakage rates that are difficult to measure.
The detection of a leak from a test part under pressure is difficult to measure when the anticipated rate of leakage from the part is low or the available time for the measurement is relatively brief. High levels of pressurization can produce sufficient adiabatic heating effects which must be dissipated before any accurate measurement can be made. The magnitude of this problem is a direct function of the mass of the pressure gas; thus, the volume enclosed by the test part and the pressure of the test gas contained within the test part may effect the magnitude and severity of the adiabatic heating effects.
Bubble testing is the most prevalent method of leak testing in the industry. It comprises pressurizing the part to be tested, submerging the part in a water bath, and looking for a stream of bubbles. Although leaks as small as 0.05 standard cubic centimeters per minute (sccm) can be detected by this method, this method suffers from major disadvantages. It is relatively slow; it demands continuous operator attention and it usually requires drying the tested part before the tested part can continue in the manufacturing process. Determination of the amount of leakage is a difficult task.
Helium mass spectrometer leak detection is the most common method used to detect very small leaks as low as 10.sup.-11 standard cubic centimeters per second (sccs). The part under test is either pressurized internally or externally with helium or a mixture of helium and air. The helium being leaked is drawn into a very low vacuum and introduced into a mass spectrometer tuned for helium. The mass spectrometer output is proportional to the number of helium ions, which is a direct measure of the rate of leakage. Helium leak detection equipment is very expensive and can require long test times for particular parts.
Another method of leak detection is the pressure decay method. In the pressure decay method the part to be tested is pressurized to a pressure determined by a supply pressure. Once pressurized, the part under test is sealed to maintain the pressure therein. A pressure sensor is attached to the part which measures the internal pressure of the part of the test. If a leak is present, the pressure of the part will begin to decay at a rate determined by the size of the leak and the volume of the part. A test operator can determine the relative size of the leak by reading the pressure at the end of the test time and comparing it to a predetermined value.
Pressure sensors used in the pressure decay method are typically gauge pressure sensors having a reference to atmospheric pressure. When the gauge pressure sensor is used at normal test pressures, the pressure change resulting from the leak test is a very small portion of the total range on the sensor, since the gauge pressure sensor measures pressure difference between the part pressure and atmosphere. Consequently, the signal from the sensor is relatively small. In order to obtain a usable reading with this system, it is often necessary to extend the test time, particularly, if large parts having small leaks are involved. In some cases this may result in unacceptably long test times.
Another method uses a mass flow leak sensor rather than a gauge pressure sensor. In precision mass flow leak testing, a mass flow leak sensor couples the test part to a non-leaking reference volume usually having substantially the same volume as the test part. Then the reference volume and the test part are pressurized to the same pressure. Both the reference volume and the test part are sealed off from the pressure supply. If a leak is present in the test part, the mass flow leak sensor measures the flow and the equalization of pressure between the leaking test part and the sealed reference volume. The difference in pressure causes gas to flow from the reference volume to the test part at a rate proportionate to the leak rate.
An alternative method to leak detection measurement using a deferential pressure sensor encloses the test part in a sealed bell jar. Leakage from the test part increases the internal pressure in the internal space within the bell jar and exterior to the test part. The increase in pressure relative to a reference pressure is measured by a transducer and converted to an equivalent leakage rate.
This method, however, is less sensitive to the effects of high test pressures and is not sufficiently accurate to detect low leakage rate particularly when short test times are employed. This is because the internal free volume of a bell jar can be large. Because the differential pressure rate involves the measurement of pressure at two different times and the time interval is a function of transducer sensitivity, this method is often not adequate in critical applications. These measurements cannot be made until sufficient time has lapsed to develop a differential pressure.
U.S. Pat. No. 5,546,789, Leakage Detection System, assigned to the assignee of the present invention, employs a sealed test fixture or bell jar which surrounds the test part and is connected to a reference pressurized reservoir. A reference bias flow is introduced into the bell jar to establish a floor or offset where any deviation measured from the introduced bias flow indicates an anomaly (or leak). The test part is pressurized and the flow between the bell jar and the reference pressurized reservoir is measured by a mass flow leak sensor. The measured flow is used to determine the leakage of the test part by taking into account the previously introduced reference bias flow. This method provides accurate measurements of leaks having low flow rates occurring over short periods of time.
While the foregoing method provides accurate measurements of leaks using a mass flow technique, in some situations the mass flow technique may not be used. For example, accurate measurement of the seat leakage of hydraulic valves is a difficult to achieve. Seat leakage occurs at very small flow rates compared to the flow rate of fluid when the valve is opened. Detection systems suitable for accurate measurements in the range of typical leakage flows are usually not robust enough to withstand the full flow of fluid when the valve is open for purging, for example. Typically, the ratio of full flow to leakage flow is approximately 250:1 or greater.
For this reason, most hydraulic leakage detection systems utilize pressure, rather than mass flow, to measure leakage. A typical pressure measurement system employs a pressure source (which may be liquid or gas), which applies pressure to the valve under test, a test pressure sensor between the source and the valve under test, a second valve and a leakage pressure transducer between the two valves. Seat leakage is quantified by measuring the change in pressure in the pipe section between the two valves after the valve under test is closed. To measure leakage pressure, the two valves are closed. Pressure is then measured in the section for a period of time and is a function of the leakage from the valve under test.
This techniques has an important drawback. The relationship between the pressure increase and the leakage is a function of the compressibility of the test fluid in the pipe section between the valves. Fluid compressibility is often expressed as bulk modulus, E, of the test fluid and is the ratio of the change in pressure to the percent change in volume, E=dP/(dV/V). Solving for dP=E(dV/V).
Bulk modulus is not constant in most practical applications; it is nonlinear in most ranges. The bulk modulus for most fluids is a function of temperature, fluid pressure and fluid purity (many fluids are hygroscopic in that they absorb water from the air). Also, bulk modulus is dependent on the proportion of entrapped or entrained air in the fluid. Even if temperature, pressure and fluid purity can be controlled, eliminating air from the test fluid is almost impossible in test situations. Since new valves for test are repeatedly being introduced into the test system, there is sometimes insufficient time to purge the entrapped air between tests. Even if additional time is taken to purge the system between tests, some air in the system, such as air in fittings, pipe threads, dead end or blind passages, cannot be purged.
There is a need for a leakage detection system and method which is independent of the bulk modulus of the test fluid.